feat: RISC-V target feature B; implies Zba, Zbb, Zbs

zba, zbb, and zbs are already stable, B is a convenient alias
for all three, so it seems reasonable to stabilize it as well.

The fourth bitmanip extension, Zbc, is not implied by B.

Author: tux3

compiler/rustc_target/src/target_features.rs

@@ -491,6 +491,7 @@ const MIPS_FEATURES: &[(&str, Stability, ImpliedFeatures)] = &[
 static RISCV_FEATURES: &[(&str, Stability, ImpliedFeatures)] = &[
     // tidy-alphabetical-start
     ("a", Stable, &["zaamo", "zalrsc"]),
+    ("b", Stable, &["zba", "zbb", "zbs"]),
     ("c", Stable, &[]),
     ("d", Unstable(sym::riscv_target_feature), &["f"]),
     ("e", Unstable(sym::riscv_target_feature), &[]),

tests/ui/check-cfg/target_feature.stderr

@@ -49,6 +49,7 @@ LL |     cfg!(target_feature = "_UNEXPECTED_VALUE");
 `avxvnni`
 `avxvnniint16`
 `avxvnniint8`
+`b`
 `backchain`
 `bf16`
 `bmi1`